To explore some of the practical functions of mental imagery, and to extend previous work on the relationship between imagery and visual perception, the proposed research will measure the effects of pattern visualization on reaction time and accuracy for detecting and making discriminations among simple visual forms. The experiments will attempt to identify the specific conditions under which imagery would facilitate or interfere with performance on a variety of visual tasks, by varying (a) whether or not instructions are given to imagine patterns just before the patterns are presented, (b) how closely the imagined patterns correspond to the presented patterns, (c) whether the task requires judgments of discrimination or detection, and (d) whether the judgments are easy or difficult. Pilot studies suggest that evidence for performance facilitation would be found only for discrimination tasks, and then only when the imagined patterns are aligned with and match the presented patterns or provide helpful context information. The results of these experiments could suggest new techniques for helping people to improve performance on stressful, demanding tasks where difficult discrimination judgments are required, and for how people might compensate for losses in visual skills resulting from age, disease, or damage to the visual system. In addition, the proposed research would reveal the consequences of uncontrolled visualization during visual perception, which could have important applications in the treatment of severe autism or schizophrenia. The research would also have major implications for current theories of how perceptual and cognitive processes interact.